Ammonium nitrate is used for a variety of applications, particularly for fertilizer compositions and explosive compositions. Selection of ammonium nitrate for either explosive compositions or fertilizer compositions is based on the characteristics and properties of the ammonium nitrate. Typically, the ammonium nitrate used for fertilizer and explosive compositions is in solid or granular form, often referred to as "prills", and the properties and characteristics exhibited by the solid ammonium nitrate are a function of the particular manufacturing process as well as any additives used during the manufacturing process.
Depending on the manufacturing process and particular additives used, the resulting ammonium nitrate prill may be classified as high density or low density. Conventionally, an ammonium nitrate prill with a bulk density of 54 pounds per cubic foot (0.865 kg/L) or more is considered high density prill. Ammonium nitrate prill with a bulk density of less than 54 pounds per cubic foot is considered low density prill, although low density ammonium nitrate will generally have a bulk density no higher than about 51 pounds per cubic foot (0.82 kg/L).
Low density prills are obtained by prilling in a prilling tower wherein liquid droplets are converted to the solid prill form by cooling a relatively dilute aqueous solution of ammonium nitrate. As a result of water evaporation during the cooling/solidification process, low density ammonium nitrate prills contain voids which increase their oil absorbance capacity. Heretofore, low density ammonium nitrate prills have been the principal form of ammonium nitrate for preparing explosives because the voids permit liquid hydrocarbon fuel to penetrate and become intimately associated with the ammonium nitrate.
In contrast to low density prills, high density ammonium nitrate prills are formed from highly concentrated molten ammonium nitrate. High density prills have not found widespread use in explosive compositions because they generally exhibit little or no oil absorptivity. Accordingly, use of high density ammonium nitrate prills have been limited primarily to fertilizer compositions.
An improved process for forming low density ammonium nitrate prills is disclosed in U.S. Pat. No. 4,749,349 to Thuring et al. These low density prills are typically used in water-in-oil emulsions in combination with fuel, water, emulsifiers, density reducing agents, and the like. However, in order to form useful explosive compositions with fuel or oil, these low density ammonium nitrate prills must contain additives which are compatible with water-in-oil emulsifiers. U.S. Pat. No. 5,034,071 to VanOmmeren discloses several additives for use with the low density ammonium nitrate prills made by the Thuring et al. process. Low density prill containing these additives is said to be useful for forming explosive compositions due to the compatibility between the external additive and the emulsifier in the oil/water composition. However, the preferred external additives of VanOmmeren tend to increase the blocking of the prills in rail cars and trucks, thereby increasing the difficulty in unloading the prills.
Handling of ammonium nitrate in powder, crystalline, or granular form has been a problem for some time not only due to the explosive nature of ammonium nitrate but also due to its tendency to undergo volume changes during storage. These volume changes are due in part to the hygroscopicity of the ammonium nitrate granules. Accordingly, many additives have been developed for ammonium nitrate prills in an attempt to improve their storage and handling characteristics. U.S. Pat. No. 3,171,716 to Diekmann et al. discloses the use of a mixture of ammonium nitrate and 0.25 to 3 wt. % sodium ions, phosphate ions and magnesium ions in the form of salts to increase the density of the ammonium nitrate and to lower its hygroscopicity.
Not only does the absorption of water increase the volume of stored ammonium nitrate, but it also causes the ammonium nitrate to become sticky on standing and form hard masses during storage. U.S. Pat. No. 3,190,774 to Wilson discloses the use of an external coating of an admixture of alkali metal salt of alkylaryl sulfonic acid and a major amount of silica-alumina for reducing the caking tendencies of ammonium nitrate during storage.
Accordingly, there is a need for a high density ammonium nitrate prill which has a sufficient oil absorption capacity so as to be useful in explosive compositions, and which exhibits good handling and storage properties.
An object of the present invention, therefore, is to provide an improved high density ammonium nitrate prill.
Another object of the invention is to provide a process for the formation of high density ammonium nitrate prills useful in explosive compositions.
Still another object of the invention is to provide a high density ammonium nitrate prill containing internal and external additives which dramatically increase the oil absorbing capacity of the prill without adversely affecting other properties of the prill, and a related process for making the same.
Yet another object of the invention is to provide a relatively simple, economical process for making an oil-absorbent high density prill useful in explosive compositions.